I'm not sure what triggers, it may just be a certain amount of time playing without fast travel/exiting the game, this but after some time iso guns, maybe all weapon isos but I've only got custom guns, some get corrupted. No amount of weapon swapping, running around etc. but if I fast travel or quit to the main menu.
I'm using a intel i5-7600K, 32GB ram. Currently I'm using the IGP as my display doesn't work with my new graphics card.
Here are before & after fast travel pictures - http://imgur.com/a/jhjU0
The iso for the particular gun is attached